1. Field of the Invention
The present invention relates to an economic water level measurement system based on a conductivity-measuring cell, to be used in a water reservoir of an appliance or any device requiring indication of the water level. The measurement system may be used when the water conductivity in the reservoir varies over a wide range of values. In particular, the water level measurement system is suitable for a steam oven.
2. Description of the Related Art
From U.S. Pat. No. 4,808,931 to Ling, it is known to use a four-electrode open-cell conductivity sensor, which measures the conductivity of ocean water through two pairs of electrodes. The first pair of electrodes is connected to an alternating current (AC) source, inducing an electric field in the area where the conductivity of the fluid is to be measured. The second pair of electrodes is used to sense the electric field generated by the first pair of electrodes. Both pairs of electrodes are built into a plastic support having a symmetric geometry, making the electric field easily predictable. The first pair of electrodes is fed with a 10 kHz AC signal having small amplitude, to prevent the chemical dissociation of the water. Any variation of the magnitude in the water conductivity causes a change in the electric field sensed by the second pair of electrodes. An electronic circuit, based on peak detection technology, and including an operational amplifier having high common mode rejection ratio (CMRR) values, converts the current flowing between the second pair of electrodes into a voltage signal, balancing the noise and polarization effects induced by factors such as the environment or dirt. The same electronic circuit also regulates the magnitude of the current source in order to maintain the RMS value of the voltage drop measurable across the first pair of electrodes. As a result, the current required for maintaining the voltage level is linearly proportional, with a coefficient depending on the geometry of the sensor and the conductivity of the fluid. The coefficient is easy to calculate, because the geometry of the sensor is defined. The amplified voltage signal is then sent to a microcomputer to calculate the water conductivity. The electronic circuit is placed in an insulated housing located close to the electrodes, therefore reducing the effects of electric noise of the surrounding environment. The closeness of the electrodes to the electronic circuit requires short connection wires, causing a voltage phase shift due to the capacitance effects of the wires. However, this effect is small and balanced by the high CMRR of the amplifier. The output of the amplifier can also be transmitted to signal processing equipment, for further processing.
From U.S. Pat. No. 6,810,732 to Shon, it is known to use an apparatus for sensing the water level in a water tub of a washing machine using a conductivity measuring cell provided with two pairs of electrodes, reference and measuring, embedded in the water tub. The conductivity of the water varies according to the type of substances dissolved in the water. The reference electrodes provide a relative reference signal that when coupled with the signal from the measuring electrodes is used to evaluate the level of water present in the water tub.
The measuring electrodes in the water tub are extended to be longer than the reference electrode, and are immersed in water when the tub is filled. The reference electrode is spaced apart from the measuring electrodes and is always immersed in water in the bottom of the tub. Independent from the turbidity of water, its electric conductivity is inversely proportional to the length of an electrode but directly proportional to a contact surface area between the water and the electrode. Thus, when the water tub is filled with water, the contact area increases, thereby proportionally increasing the electrical conductivity of the cell. A fixed direct current (DC) voltage is applied to both electrode pairs, providing a voltage signal correlated with the electrical conductivity of the water cell. This signal is proportional to the voltage measurable across a resistor, in series with the electrodes, when the current flows through them. The two voltage signals from the electrodes are read by a microprocessor. The reference electrode pair provides a reference value for water level measurement, while the measuring electrode pair provides the measured conductivity value, which varies according to the water level. The water level is determined by calculating the difference between the reference and measured values of the conductivity. This technical solution is particularly effective in water level detection in turbid water, where the conductivity changes according the substances dissolved in the water.
The above described device is typically used in washing machines, and its performance is limited when the conductivity of the water is low, as in the case of distilled water. Moreover, this device does not allow complete emptying of the water reservoir and thus requires additional space in the lower side of the reservoir.
Also known in the prior art are water level measuring devices using two electrodes in an open conductivity cell, based on an LC oscillation circuit and peak detection circuit. These devices often fail to precisely measure the conductivity because they are relatively unstable due to temperature variation and long-term wear and tear.
Also known in the art are detection methods that include variation of the frequency of the ac source signal to remove coupled signal interference or to get indication of the scaling on the electrodes.
The prior art also discloses how to detect the presence of water by a measuring cell using water having good conductivity characteristics (washing machine water or seawater, both in the range of hundreds of μS/cm). In this solution, the electrodes are located near the electronic detecting circuit. A problem arises when the water in the reservoir has low conductivity (lower than 1 μS/cm, as in case of distillated water), and when the measuring circuit cannot be placed near the electrodes due to an unsuitable surrounding environment. Typically, an unsuitable environment is characterized by high temperature and humidity and by the presence of electrical noise.
An unsuitable environment can be found, for example, in a steam oven provided with a water reservoir for steam generation, which is located close to the hot cavity of the oven. In this environment the temperature and humidity conditions are severe, and the space to position the electronic devices is small, making it difficult to cool the electronics. In such a case, the electronic circuit may be positioned in an insulated and protected environment, such as the control panel area. This area can be additionally cooled with fresh air to better protect the electronic components. In this case, the control panel is probably far from the electrodes that are positioned within the reservoir, and the connection between the electronic circuit and said electrodes requires long wires, rated to sustain the conditions of the surrounding environment.
Moreover, in a steam oven and other consumer appliances, it is possible to face the problem of water having low conductivity. This is typically the case when consumers fill the reservoir using distilled water in an attempt to reduce the scaling due to calcium contained in the water, with the purpose of extending the lifetime of the appliance. In this case, the resistive value of distilled water in the reservoir is comparable with the parasitic capacitance of the wires connecting the electronic circuit to the electrodes. Thus, the resistance of the water and parasitic capacitance of the wiring introduce a large error in the measurement of the conductivity, because the circuit is unable to distinguish between the active and reactive components. More specifically, the capacitance of the cables connecting the measuring circuit to the electrodes can be in the 100 pF range, which at a frequency of 1 kHz, yields a capacitive reactance of 1.5 MΩ. In the same way, the resistance of the water in the reservoir ranges from a few hundred MΩ for distilled water, to KΩ values for water with salts.
Finally, the electrical noise of the environment is an additional parasitic component, causing further degradation of the conductivity-related signal. This undesired effect becomes more important when long cables, more than 30 cm long, are used. To reduce the negative impact of the noise, the art teaches to locate the electronic board close to the electrodes or to use shielded wires, although the shielded wires show a capacitance effect between the two electrodes. These solutions are not always applicable in a hot environment, and they are expensive for large-scale production, as in the case of consumer appliances.
It follows that it is not possible to measure the presence of very low conductivity water, such as distilled water having a conductivity of less than 1 μS/cm, in a reliable and economic way, if a special circuit is not used.
Additionally, it has also been observed that some users fill the water reservoir of appliances using mineral water that presents high conductivity values (more than 1000 μS/cm), for which a standard circuit is suitable.
Thus, there is a need to have consumer appliances that can provide a reliable conductivity measurement over a wide range of water conductivity values, and that is economical for mass production.